In short: Google Pixel 3 and Pixel 3 XL smartphones that are linked to Verizon will not work on other carriers unless they are activated through Verizon first. This is based on confirmation given by a Verizon representative to a number of media outlets including The Verge who confirmed this is not a policy specific to the Pixel 3 line, but one the company introduced some time ago as a means to reduce the accessibility for stolen smartphones to be sold or used without first having been activated on Verizon's network.
As this is not a feature designed to lock users down to Verizon per se, the unlocking of the Pixel 3 does not require the same sort of unlocking procedure that might have previously been encountered by consumers when looking to switch networks. So there is no lengthy periods to wait or significant hurdles to pass, or phone calls to make before you can get a device unlocked. However there is still likely to be a waiting period to some degree, albeit a very short one. As in line with how Verizon previously described the process, once the Pixel 3 or 3 XL has been activated on Verizon's network, it will become unlocked from then on, or at worst within the next 24 hours through a software update. Therefore besides waiting on the software update to arrive and applying it when it does, there is nothing else a user will need to do to unlock a Pixel 3 or 3 XL.
Background: The Google Pixel 3 and 3 XL mark the latest entries from the house of Google (they have only gone on general sale starting today) and are the company's flagship options designed to showcase the core benefits of Android while also providing a premium smartphone camera experience – and especially when it comes to the front-facing (aka selfie) cameras. However, getting hold of a Pixel phone has not always been the easiest of tasks and especially during the initial weeks and months of release. As with this being a new line from what is still in effect a brand that's relatively new to the smartphone game, there has in the past been issues with stock and fulfilling orders from source.
On top of this, there has also always been the wider issue of availability at the mobile operator level. As ever since the first phone from the Pixel series launched, they have remained exclusive to Verizon. Up until now, that has remained more a case of marketing spiel than anything else as the Pixel phones have always been highly carrier compatible meaning you could buy a Pixel phone from just about anywhere and put any SIM card in and immediately start using it. Technically, that has not changed as the Pixel 3 phones are just as capable of working with all major US carriers as the previous models were. It's just these version now require this brief activation first. While this is not technically an example of SIM-locking, at the same time, it is SIM-locking.
Impact: Although this is a policy that is seemingly in place for all devices now sold through Verizon, the issue is a little more pertinent when it comes to the Pixel 3 line simply due to the general lack of availability at the carrier and store level. So while the two phones do support all necessary bands to work with the major carriers in the US, the need to first authenticate either of the phones might prove problematic for some buyers, and especially those who would prefer to buy the Pixel 3 or 3 XL from a store instead of directly from the Google Store. Opting to go the non-Google route is not necessarily a seller favoritism issue either, or one based on having to wait for Google to ship units (which is often an issue to be honest), as one of the main benefits of ordering through Verizon (or any carrier or store for that matter) is that more often than not you can secure a product at a cheaper price than you might otherwise. For example, Verizon is currently running a promotion where those who buy a Pixel 3 or 3 XL can get a second one for free. Which makes it a much more affordable option to go the Verizon route compared to the Google Store route and especially for those in the market for more than one smartphone. In fairness, taking advantage of the BOGO through Verizon is dependent on receiving the free value in the form of bill credits and that's not the same as buying a phone outright – and so it's not directly related to the unlocking issue at hand. However, it still highlights the point that carriers and stores are usually keen to attract business with better deals than you might get from the manufacturer and that's likely to be the case when it comes to buying what would be considered an unlocked Pixel 3 from Verizon in the coming months.
Of course, Best Buy has always remained an alternative option when it comes to the Pixel line and while that is still true today, that's not to say the issue is any easier this time around. As the Pixel 3 and 3 XL models sold through Best Buy also happen to be the Verizon models and therefore will also first need to be activated on the operator's network before they can be unlocked for use on another network. The end result being the only way a consumer can buy a Pixel 3 or 3 XL today and be sure that it will definitely work on a non-Verizon network without having to go through the authentication process is to purchase directly from the Google Store. As a side note, this will also mean that anyone who purchases through a re-seller will want to make sure the device they are purchasing is in fact a non-Verizon model. Or at the very least, be able to access an active Version SIM card for a short period of time and until the phone has completed the authentication process and the unlock update has been received.